A Brave New World

Tips from Hemmings Classic Car

Company: General Motors Corp.
Job: Tuned Port Injection
In the autumn of 1984, America was getting ready to re-elect President Reagan, the "Star Wars" Strategic Defense Initiative missile system was on the way and Chevrolet and Pontiac dealers were about to introduce the most advanced fuel delivery system the U.S had ever seen: General Motors' Tuned Port Injection. It quickly became known as TPI.
Increasingly stringent federal emissions and fuel economy standards meant that the industry had to completely rethink fuel delivery and ignition control logic. The GM throttle-body injection (TBI) system that was featured in a previous installment of Mechanical Marvels (HCC #83, August 2011) was only a stopgap in the transition from the carburetor. The problem with TBI was that it was wet flow--fuel and air coursed through the intake manifold runners. This meant that there was an inherent lag in the system's ability to alter the mixture ratio based on data from the oxygen sensor. A more responsive method was required. Placing a fuel injector at each cylinder (port fuel injection) would be necessary.
Individual port fuel delivery was nothing new to the marketplace. Many domestic and foreign manufacturers used this design, which was fundamentally based on the original theory brought to market by the Bosch Corporation of Germany. But GM's TPI united design elements that were not previously incorporated into one system.
More Than a Name
Most enthusiasts misunderstood the letter "T" in the TPI name. They believed that the system was synonymous with electronic fuel injection; it was not. Instead, it described the theory applied to the intake manifold design. Many would incorrectly identify any port EFI system as a TPI.
When studying an intake manifold, you can apply two theories: tuned and un-tuned. In simple terms, a tuned induction system has a defined and narrow RPM range where it is very effective at filling the cylinders with charge. In contrast, an un-tuned induction system does not enjoy the so-called sweet spot. It is effective over a broader RPM range, but is not exceptionally efficient at any one operating speed.
The total intake manifold flow path is identified as the runner length. It begins at the backside of the throttle butterfly and ends at the intake valve. A concern with a wet flow fuel delivery method is that the runner length is limited due to packaging and the need to keep the fuel in suspension in the air stream. Thus, the ability to tune the intake manifold for a desired performance characteristic is available, but limited.
As intake manifold runner length is increased, the engine's ability to produce low-speed torque is greatly enhanced, but at the expense of high-RPM horsepower. The shorter the intake path, the greater the high-RPM horsepower--but the torque output is sacrificed, especially at low engine speeds.
Remember, the consumer buys horsepower but drives torque. Thus, an engine with prodigious amounts of low- to mid-range torque will feel very lively and offer excellent performance. In addition, a high specific torque output would allow for a lower rear axle ratio and tighter torque converter, which will result in better fuel economy.
This basic fact of engineering was not a revelation, but automakers had only a limited ability to increase runner length, especially on a V-shaped engine. There was, however, a little more freedom with an inline engine design.
In the 1960s, Chrysler brought to market a 413-cu in V-8 with a cross-ram manifold that enjoyed the longest runner length available until the GM TPI system came around. The Chrysler design placed one carburetor by the radiator and the other by the brake master cylinder. This engine was a good performer at full throttle, but suffered from poor driveability and cold start balkiness due to the long distance the fuel needed to travel and its impact on the signal created in the booster of the carburetor. It remained in production for only a short while.
GM faced none of the issues that Chrysler did in trying to increase runner length when creating the TPI. Only air was traveling through the intake manifold; the fuel was introduced at the juncture where the intake manifold met the cylinder head runner. The total runner length of the TPI system was about 30 inches when counting the cylinder-head port. In contrast, the same engine with a factory carburetor-style un-tuned intake manifold had a total air path of approximately 14 inches to the valve from the plenum. The TPI system quickly became known as a torque monster, especially when compared to other engines of that time, which were choked due to smog restrictions.
Included Components
The TPI system used the following parts:
Engine Control Module - The ECM was the electronic brains of the system and controlled everything.
Rochester Products Aluminum Fuel
Rail and Pressure Regulator-Fed Fuel Injectors - Eight Bosch-sourced high-impedance fuel injectors each moved 22 pounds of fuel per hour.
Fuel Pump - The in-tank Rochester/AC-
Delco roller-vane electric fuel pump produced approximately 45 PSI of fuel pressure and kept flow constant despite engine load.
Mass Air Flow Sensor - The MAF was connected between the air cleaner assembly and throttle body. It measured the volume of incoming air flowing across an integral sensing wire. Then it sent an analog voltage signal to the ECM that used this data to determine load on the engine for fuel delivery and ignition control.
Throttle Position Sensor - Mounted on the throttle-body and attached via a lever to the throttle plates, this 0- to 5-volt sensor provided throttle angle input to the ECM.
Coolant Sensor - Mounted in the water jacket of the intake manifold, this sensor was a thermistor. As its temperature increased, the resistance dropped. This is the opposite of a resistor. The impedance increased with thermal load. The coolant temperature was used to trim the fuel and timing commands that were sent to the engine from the ECM.
Air Charge Sensor - This sensor resided in the bottom of the intake plenum and was a trim input to modify fuel and ignition delivery as incoming air became heated.
Oxygen Sensor - Placed in the exhaust manifold on one side of the engine, this single-wire sensor was the auditor to confirm that the air/fuel ratio was being delivered at a rate of 14.7:1 under most operating conditions. This is the mixture strength that is required for the best chemical conversion rate in the catalytic converter.
Knock Sensor - Fastened to the engine block between two cylinders, this was a piezo accelerometer. In lay terms, it could be considered an electronic tuning fork. When engine knock occurred, it produced an output voltage and the ECM would retard ignition timing to quell detonation.
Electronic Timing Distributor - The ignition distributor had no centrifugal weights or vacuum advance. Instead, a module would work in conjunction with the ECM to control the ignition advance based on the input from the sensors.
Cold Start Injector and Thermal Time Switch - A ninth injector was placed in a special passage cast into the intake manifold, along with a dedicated switch in the coolant jacket. This injector would be evoked on cold start based on the water temperature. It would stop providing fuel later as signaled by the module in the distributor. The engine was considered running once the RPM reached 400.
Design Limitation
When introduced for the 1985 model year, the TPI was to find its home in GM's F- and Y-body model lines. These were the Chevrolet Camaro/Pontiac Firebird and Corvette, respectively. The Corvette was completely redesigned for the 1984 model year, featuring a twin throttle-body injection system that was marketed under the name Cross-Fire Injection. To aid in aerodynamics and produce a top speed in excess of 150 MPH, the new, fourth-generation Corvette had a very low hood line. This meant that the designers of the TPI needed to make the system with 30-inch-long runners fit under a low hood.
This limitation meant that the entry angle from the intake manifold base to the cylinder head had to be very flat. That would, in turn, impede airflow, since the best path is a straight shot to the intake valve.
Another problem was that the long runner length allowed for a high level of heat transfer from the aluminum components to the incoming air. A general rule states that for every 10 degrees F the air is heated, one percent power is lost, and the propensity for detonation becomes much greater. The low hood closed on the TPI plenum and caused underhood temperatures to be much higher than in any other production vehicle of that time, and very possibly to date. The TPI system's sprightly performance when cold was quickly eroded on a hot summer day.
At that time, most, if not all, gasoline was formulated to work with a carburetor, and many port EFI systems on both foreign and domestic cars suffered from carbon deposits forming on the pintle of the fuel injector. The TBI system, with its injector in the cooler ambient air stream above the throttle plate, did not suffer from this problem. When the injector became carbon-coated, idle quality and overall performance suffered greatly. Thus, an engine that was supposed to be powering America's only sports car was back at the dealer with a poor running condition in short order. Though the problem was not GM's fault, many dirty injectors were changed under warranty. In 1986, GM, working with Rochester Products, produced its own injector, known as the Mul-Tec, which used a ball and seat in lieu of the Bosch pintle; it was much less prone to failure or problems due to fuel design and composition.
The German-made Bosch MAF sensor also proved to be problematic, with a nine out of 10 failure rate in the first few years of use. When the sensor failed, the ECM would impart a "limp home" strategy that got you where you needed to go, but with a very poor running engine. The heated wire sensing element would fail prematurely.
As with any great leap forward in technology, the foray into port EFI was met with both success and failure. The TPI system, when not faced with dirty injectors and only nominally hot air, was a stunning performer, especially when mated to the excellent Turbo Hydra-Matic 700R4 transmission and torque converter. The system eliminated the cold start injector for the 1989 model year and the Bosch MAF in 1990. Instead, the starting fuel was administered with the eight injectors and a manifold absolute pressure sensor that monitored engine vacuum determined load. These changes, when combined with the GM Mul-Tec injector, made for a very reliable and excellently performing vehicle under almost all operating conditions.
For the 1992 model year, the L98 TPI V-8 engine and system were replaced with the new LT1 in the Corvette (1993 for the F-body). Interestingly, GM took the opposite approach with intake manifold design and went to an un-tuned, very short (approximately 11 inches to the valve seat) intake manifold.
The DNA of the GM TPI system, engine control logic and fuel delivery technology can be seen in some form today in engines produced around the world. It's a true marvel that never got its due.

This article originally appeared in the May, 2013 issue of Hemmings Classic Car.